AGC Misunderstanding

I noticed AGC has no affect when using a program like WJST-X. I switched AGC off and brought the slider all the way to zero. No audio from the speakers but there was no impact on WSJT-X's waterfall nor on its ability to decode. I'm guessing AGC is really just for listening?

While doing the pre-post search on this question I came across two videos from Steve N4LQ. One of the two showed how he adjusts AGC and I think it is really helpful. The other kind of complained about AGC on the Flex and I found that one confusing. The links have been posted here before but I'll repeat them.

Aha moment - in the WSJT-X waterfall, you have "flatten" checked, which will cause the display to try to compensate for uneven levels in the passband. It's like an AGC for the waterfall. However, decoding doesn't take place from the waterfall values, so the "thermometer" gauge is your best bet - adjust rig levels for 30 dB there. AGC to the noise "knee" and volume to get the level.

Just a quick follow-up. Many thanks for helping me to understand what was going on. I've been working with AGC setting recently and think I can properly set it for any mode. I've noticed, when set right, strong signals can pretty much wipe out noise but leave weaker signals intact.

In this screen shot of WSJT-X displaying JT-65 the first minute (lower portion) there's a strong signal around 600 Hz. Background noise dropped off almost totally and the other signals within the display sounded and decoded fine.

You can see the next 12 seconds with no signal. Noise came up on the display and in the audio output. Then the next minute starts with no particularly strong signals. Noise dropped a bit but again, copy of all signals was perfect. Relative volume in each case was easy listening. The strong signal was hardly any louder than any other signal.

I'm wondering (and I'll experiment), if the second minute trace indicates I should drop my AGC threshold down a little bit more.

On CW with a decently strong signal I get the same results. The noise just drops out and I'm left with a pleasant sounding CW note. I practiced this on the QRP Fox Hunt last Thursday and got both foxes within 25 minutes. Generally I only get one or no fox.

I kind of wonder why this AGC action wouldn't be made visible in the panafall of SmartSDR. I suppose it has to do with where in the signal flow WJST and SmartSDR is getting its data to display.